A food Calorie represents the amount of energy that will raise the temperature of one kilogram of water one degree Celsius.
What's In A Calorie?
And here's one way scientists measure the Calories, or the amount of energy, in foods. The food goes into something called a bomb calorimeter.
It's a very well-insulated box filled with oxygen and surrounded by water. Once the food is sealed inside, it's ignited electrically and burns quickly to ashes. The temperature of the water surrounding the box must be measured before and after the food burns.
If the box is surrounded by one kilogram of water, then each degree the water heats up represents one Calorie in that food.